This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Polyporus squamosus agglutinin (PSA) from mushroom Polyporus squamosus is a 28 kDa lectin which agglutinates human A, B, and O and rabbit red blood cells. It binds [unreadable]-D-galactosides and exhibits high specificity and affinity towards nonreducing terminal Neu5Ac-[unreadable]-(2,6)-Gal-[unreadable]-(1,4)-Glc/GlcNac (6-sialylated type II chain) of N-glycans. The strict specificity of PSA for [unreadable]-(2,6) linked sialic acid residues makes it a valuable tool in biomedical and glycobiological studies. The structure of PSA and its binding interaction with its ligand is unknown. PSA shares very low sequence homology with known protein structures. Native PSA crystals were grown with the ligand 6-sialyllactosamine and X-ray data were collected at 1.7 [unreadable] resolution. Derivative heavy atom crystals of PSA are being grown to help solve the crystal structure of PSA. To obtain more information about the carbohydrate recognition domain of PSA, a truncated, active, C-terminal deletion mutant (PSA-D1) has been constructed.